Made the line breaks in header values encoded with q-encoding be followed
by a tab character to prevent being interpreted as a new header that could
be used for abusing mailing scripts to send messages to unintended
recipients.
Added the functions GetMessage and GetMessageSize to retrieve the current
message headers and body text and size without sending the message.

Prevented data corruption when attaching files to a message and PHP option
magic_quotes_runtime is On.
Return a more explicit message when setting the Return-Path header and PHP
option safe_mode is On.

Added the SetBulkMail function to hint the class when a bulk mailing
delivery starts and ends so the class can optimize the delivery method
to make messages deliveries to many recipients more efficient.

Added information on how to prevent receiving temporary delivery failure
warning messages.
Added information about the sub-classes documentation that have more
details on solutions to deliver messages to many recipients more
efficiently.

Added support to the Disposition parameter for the file parts so the
applications can hint the receiving mail programs about whether the files
embedded in the message are to be displayed inline or are attachments.

Updated the e-mail validation regulation expression to allow addresses with
.museum top level domains.
Tried to use mail() 5th argument to set the return path header if possible
or otherwise fail.
Made public functions fail if there was a previous error so it is now
possible to call several functions and only check if there was an error
in the last call.